Rev: 01
P/N: 95343
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applied to one of each electrode pair, and the amount of current required to maintain that voltage is measured.
As the conductivity of the water increases, the current increases.
The MP25 reports conductivity as Specific Conductance (conductivity standardized to 25°C). The conductivity
sensor is also used to calculate Total Dissolved Solids (TDS) and Salinity from the conductivity value.
The zero point for the sensor is set electronically, so you need only set the “slope” point for calibration:
1.
Fill the calibration cup with your conductivity standard to cover the conductivity sensor. Tap gently on
the cup to make sure there aren’t bubbles trapped in the conductivity sensor.
2.
Follow the MP25 software calibration instructions.
pH
pH is measured as the voltage drop across the glass membrane
of a pH electrode. A reference electrode is used to complete
the voltage-measuring circuit. The pH glass is specially
formulated to absorb water so that ions (particularly H+ and
OH-) in the water are attracted to the glass to offset the ionic
constituency of the pH electrode’s internal electrolyte. As a
result, there is a charge separation across the glass, and that’s
the voltage we measure. pH readings are automatically compensated for temperature.
You can choose a two- or three-point pH calibration. The two-point calibration is most common, using a pH 7
buffer and a pH 4 buffer if expected values are below 7 or pH 10 buffer if expected values are above 7. If you are
measuring in waters whose pH might range both above and below seven, you can increase your accuracy slightly
by choosing a three-point calibration (the third buffer should be on the other side of seven). To calibrate pH:
1.
Rinse your sensors at least twice with the pH buffer you’ll use for calibration.
2.
Fill the calibration cup with enough buffer to cover both the pH and reference electrodes.
3.
Follow the MP25 Control Software calibration instructions.
4.
Repeat steps 1, 2, and 3 if you choose to calibrate with one or two more standards.
pH electrode maintenance is minimal and only requires occasional cleaning of the glass bulb surface with a soft
cloth or soft cotton swab and soapy water. Do not use anything abrasive. The really important part of pH
maintenance is refilling the reference electrode.
pH/ORP Reference Electrode
The key to reliable pH, ORP, and ISE measurements is a well-maintained reference electrode. The MP25 system
uses a serviceable reference electrode with a replaceable electrolyte and reference junction. This combination
greatly extends the life of the pH/ORP probe as compared to probes that use integral reference electrodes with
gelled electrolyte. The free-flowing junction design also responds better in low conductivity waters.
Reference electrode maintenance is as follows:
